Potentially the greatest natural threat to eelgrass beds is the
periodic outbreak of wasting disease, which appears to principally affect sublittoral beds
of Z. marina. Between the 1920s and mid-1930s, formerly extensive eelgrass beds on
both sides of the Atlantic experienced significant declines in the first recorded major
outbreak of the disease. By the end of this outbreak, wasting disease had been reported
throughout western Europe. The narrow-leaved form of Zostera (presumably Z.
angustifolia) was less affected by the disease, while Z. noltii did not appear
to be affected at all (Rasmussen, 1977).

The symptoms of wasting disease are the appearance of rounded, dark
brown spots on the leaves, which coalesce until the leaf is completely blackened. The
leaves die and detach from the main plant, the regenerative shoots decay and after two or
three seasons of this defoliation, the rhizomes discolour and die. The final stages of
this disease can be devastating, with up to 90% of the plants being lost and the bed being
laid bare.

The indirect effects of the disease were also severe. A variety of
characteristic and representative species declined or disappeared, some fisheries declined
and a number of beaches and sandbanks, previously protected by eelgrass, experienced
increased erosion. The food supplies for overwintering wildfowl (wigeon, Brent geese and
swans) were reduced, forcing the birds to migrate to different feeding grounds

Recovery did not begin until the mid-1930s and has generally been slow.
A further decline in the Dutch Wadden Sea was reported in the 1970s (Den Hartog &
Polderman, 1975; Polderman & den Hartog, 1975; van den Hoek et al., 1983). In the
early 1980s, wasting disease reappeared on the east coast of North America (Tubbs, 1995;
Short et al., 1986; Short et al., 1988). Between 1987 and 1992, symptoms of wasting
disease appeared in several populations in north-west Europe, including estuaries on the
southern coast of England and the Isles of Scilly (Fowler, 1992).

The causes of wasting disease have been debated since the 1920s - 1930s
epidemic and several causative factors were suggested, including a number of fungal,
bacterial or protozoan pathogens. After the 1980s outbreak, research in America identified
the pathogen as the fungus Labyrinthula macrocystis. Muehlstein et al. (1988, 1991)
showed that Labyrinthula does not generally cause disease in low salinities,
explaining why UK populations of the intertidal Z. angustifolia and Z. noltii appear
to have been relatively unaffected by wasting disease.

It is likely that the causative factor, presumed to be L.
macrocystis, persists at low, harmless levels within Zosteramarina populations
between epidemics. The reasons for the disease outbreaks are not fully understood (Giesen
et al., 1990a, b), but it is possible that Zostera plants only succumb when
stressed by other environmental factors such as low levels of insolation, increases in
water temperature, or pollution (Short et al., 1988). The disease may occur periodically,
in an unredictable long-term cycle whose triggering factors remain to be identified.

Wildfowl grazing

Several studies in Britain have monitored changes in eelgrass
populations in relation to grazing by overwintering wildfowl, particularly wigeon and
Brent geese. Zostera is an important food source for wildfowl, providing a
concentrated and nutritious food supply that quickly replenishes energy reserves expended
during migration. As overwintering wildfowl numbers can fluctuate from year to year, often
related to weather patterns, the grazing pressure on Zostera can be highly
variable. When migrant birds arrive at their overwintering site, they generally
preferentially feed on eelgrass and only switch to algae when the Zostera resource
becomes exhausted. Wyer etal. (1977) suggested that Z. noltii is the most
important of the three species. It retains its leaves well into the winter, unlike the
other two species which begin shedding their leaves in the late autumn. As Z. noltii is
found highest up the shore, the low water grazing period is longer.

Wigeon nip off the eelgrass, blade by blade, without much waste. Brent
geese tear up parts of the plant and the material they do not consume floats away on the
surface. However, when they stop feeding directly on the eelgrass beds due to the rising
tide, they may later locate and feed on this floating reserve material
(Butcher, 1941a). Swans tear up large quantities, with the rhizomes attached, but do not
consume all the plant material disturbed. Madsen (1988) found that geese feed
preferentially on above-ground material and only shift to the below-ground material at
lower Zostera densities. However, in Strangford Lough, Portig et al (1994)
found that the impact on the below-ground biomass occurred as soon as birds arrived, as
the Zostera occurs on thixotropic mud which liquefies on disturbance, making it
easier for the birds to paddle and dig for rhizomes.

Grazing wildfowl can consume a high proportion of the available
standing stock of Zostera. Portig et al. (1994) found that in Strangford Lough, 65%
of the estimated biomass (~1100 tonnes fresh weight) of Zostera was consumed by
grazing wildfowl but that up to 80% was disturbed by their feeding activity. The
above-ground biomass (~330 tonnes fresh weight) was reduced by 93% while the below-ground
biomass (~770 tonnes fresh weight) was reduced by 74%. Tubbs and Tubbs (1983) reported
that Brent geese grazing resulted in the cover of Z. marina and Z. noltii
being reduced from 60-100% in September to 5-10% between mid-October and mid-January.
Jacobs et al. (1981) estimated that grazing wildfowl consumed 50% of the total standing
stock of Z. noltii at Terschelling in the Dutch Wadden Sea. Madsen (1988) reported
that in the Danish Wadden Sea, dark-bellied Brent geese consumed 91% of the Zostera
biomass in consecutive years.

At Lindisfarne, Northumberland, Percival (1991) reported that grazing
pressure did not affect the percentage cover of Zostera until late winter and that
most of the loss appeared to be due to other factors, particularly wave action during
storms. It appears that Zostera can recover from normal levels of
wildfowl grazing (Charman, 1979; Madsen, 1984; OBrian, 1991; Ranwell, 1959; Tubbs
& Tubbs, 1982), but if a bed is stressed by other factors it may be less able to
withstand grazing pressure. An example of this was reported by den Hartog (1994b) who
found that Brent geese may have removed the few remaining healthy plants that survived
after beds of Z. marina / Z. angustifolia in Langstone Harbour had been overwhelmed
by growth of the alga Enteromorpha.

Epiphyte grazing

It was noted elsewhere that epiphyte grazers such as Hydrobiaulvae
can contribute to the health of Zostera plants by removing the algae which foul the
eelgrass leaves. Any factors (natural or anthropogenic) which reduce grazer populations or
cause increased proliferation of algae may therefore have an indirect adverse impact on
the Zostera bed. The factors most likely to cause such changes are pollution
incidents (causing grazer mortality) or excessive nutrient enrichment (causing
eutrophication). These processes are most likely to occur as a result of human activities
and will therefore be discussed more fully in the section on impacts by human activities.